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The goal is to fly around the world using no fuel other than sunlight. But first a prototype of the solar plane to circumnavigate the globe will fly across the U.S.—taking off on May 3 and potentially proving that airplanes can fly long distances on nothing more than sunshine.

"I come from a family of explorers," explains Bertrand Piccard, the first man to fly a balloon around the world and chairman of this Swiss effort, dubbed Solar Impulse. "Solar Impulse was the logical consequence after that balloon flight around the world [which consumed nearly four metric tons of propane] in order to make a big adventure that would inspire people about innovation and the protection of natural resources."

A deep-blue array of 12,000 silicon solar cells stretches across the more than 60-meter wingspan, which is nearly as long as a Boeing 747’s. At 1,600 kilograms, the airplane weighs less than a compact car. The electricity-producing solar cells themselves are only 135 microns thick—the width of a human hair—yet they can convert more than 20 percent of the incoming sunlight into electricity, and more than enough for takeoff, powered flight and a daily recharge of the batteries that keep the plane aloft through the night.

Takeoff requires 10 horsepower from each of four electric engines, which is supplied by the solar cells and pre-charged lithium ion batteries. The engines are quickly throttled back to seven horsepower to climb and then as little as 2.5 horsepower per engine is required at cruising altitude where the plane flies at roughly 80 kilometers per hour. The plane climbs to 9,000 meters by mid-afternoon when the sun shines most abundantly and then, to conserve power, slowly descends to 1,500 meters over the course of the evening where it flies on battery power until the next sunrise, when the cycle repeats anew. "In order to stay airborne until the next sunrise it has to be that big and that light," Piccard explains. "We have calculated what we need in terms of energy, so that gave us the surface and it gave us the wing loading."

At least a quarter of the plane’s weight is the 90kilowatt-hourlithium ion battery pack. "If you want to spend the entire night airborne and almost perpetually in the air, then you need lots of batteries and a big wingspan," Piccard notes.

The wings, fuselage and gondolas that bear the engines are made from composite materials, including strong but light carbon fiber. "We just have carbon fibers at places where the forces are," Piccard notes. "If no forces, it's empty." And the next iteration of the plane, capable of flying around the world, will have 20-meter-long carbon-fiber sheets that weigh just 25 grams per square meter, three times lighter than a sheet of paper. Then there is the special insulation (also to be used in refrigerators) required to keep the pilots in a comfortable environment at an altitude of more than 8,200 meters—the temperature swing could be from –40 degrees Celsius to 30 degrees C. "When we talk about energy savings, it's very boring. People are not excited with it," Piccard says. "The airplane is an exciting way to promote these technologies."

The biggest challenge of these long flights might by psychological rather than technical. For stretches of up to 24 hours, the pilot has to stay alert enough to fly the unwieldy solar plane. "Flights can be above 20 hours without problem," Piccard argues. (In 2010 the team completed a 26-hour test flight). Flying it won’t be easy: the plane wasn’t built to handle turbulence, let alone inclement weather, and the massive wingspan makes banking a constant danger. To help the pilot, the team developed a computerized alarm based on vibration to signal when the plane exceeds 5 degrees of bank angle. "If you exceed [that] on the left side, [your] left arm starts to vibrate," explains Solar Impulse CEO and co-pilot Andre Borschberg, prompting an almost unconscious and instantaneous response. "You can almost do it with closed eyes."

The two pilots plan to use self-hypnosis, yoga and meditation in their effort to fly around the world—and they will need iron control of bodily functions. "When we take off, there are full bottles of water on the right side and empty bottles on the left side," Piccard explains. “When we land, it’s the opposite.”

After taking off from Moffett Field near San Francisco, the solar plane will make stops along the way in Phoenix, Dallas, Saint Louis and Washington, D.C., before landing in New York City sometime in July. The plan is to keep each flight leg under 24 hours, although the self-powering plane can fly longer than that. Because there is only room for one in the cockpit Piccard and Borschberg will swap duties—all in preparation for the extended flights that will be required for the round-the-world trip. "To fly around the world we will have to fly many days and nights," Borschberg says. "The Pacific Ocean flight will be at least five days and five nights."

The solar airplane program, which cost some $150 million and took 12 years to design and build, is meant to showcase technologies that could be used in other applications—refrigerator insulation, say, or solar-powered drones that stay aloft for extended periods. But solar airplanes won't be taking passengers anytime soon. Piccard notes that Charles Lindbergh flew alone across the Atlantic, and it took another 25 years for a conventional aircraft to carry 200 passengers. "Solar Impulse is just the very beginning of a long haul for solar airplanes."